The technical portion of the work performed by an optician consists in mounting a pair of ophthalmic lenses in a frame selected by a wearer. Such mounting is made up of five main operations:                reading the outlines of the bezels of the rims of the frame selected by the wearer, i.e. the outlines of the grooves going around the inside of each of the rims of the frame;        centering each lens, which consists in determining the position that each lens is to occupy in the frame so as to be appropriately centered relative to the wearer's eye;        feeling each lens, which consists in determining the coordinates of points characterizing the shape for the outlines of the lenses; then        shaping each lens which consists in machining or cutting its outlines to the desired shape, given the defined centering parameters; and finally        beveling which consists in forming a bevel that is to hold the lens in the bezel included in the frame.        
In the context of the present invention, it is the first operation of reading the outlines of the bezels of the rims of the frame that is of interest. Specifically, the optician needs to feel the inner outline of the rims of the selected eyeglass frame in order to determine accurately the coordinates of points characterizing the outline of the bottom of the bezel. Knowledge of this outline enables the optician to deduce the shape that is to be presented by each of the lenses once they have been shaped and beveled so as to enable them to be mounted in the frame.
The particular purpose of this operation is to follow very exactly the bottom of the bezel included in each of the rims that is to be read so as to be capable of storing an accurate digital image of the shape of the bezel.
For frames that are strongly “curved” and “skewed”, i.e. strongly cambered and twisted, merely pressing the feeler against the bezel orthogonally to the axis of rotation of the feeler does not enable the feeler to follow the bottom of the V-shaped bezel accurately. In particular, when the “skew” or twisting of the frame is very considerable, and one of the side faces of the bezel is steeply inclined, then there is a risk of the feeler escaping from the bezel by sliding along that side surface.
By way of example, document U.S. Pat. No. 6,325,700 presents a device for following a bezel, which device is designed to mitigate this problem of escaping from the bezel. The device comprises an outline reader appliance in which a frame is placed, the appliance controlling the position of the feeler as a function of the curvature of the path followed by the feeler.
The drawback of such a device is that in certain portions of the rim of the frame, and in particular in its temple portions, the curvature of the path followed by the feeler along a first axis (normal to the general plane of the rims of the frame) is not large, unlike the skew of the frame. As a result, in certain portions of the rim, the control over the position of the feeler does not respond effectively to the skew of the frame.